The invention concerns methods for producing plastic laminates, especially those for printed circuits.
As is widely known, plastic laminates are sheets obtained by stable association, usually made by pressing, of several layers of plastic materials on a backing of paper, fibreglass fabrics, or other kinds.
The plastic materials may be of phenol, melamine, epoxy, polyester, silicon, fluoride or others.
In the production of printed circuits a metal lamina, usually of copper, is pressed onto one or both their sides.
A pile of packages, all substantially the same, is formed each comprising a number of layers impregnated with the plastic materials, the copper lamine being placed externally. A sheet of metal, such as stainless steel, is laid between one package and another, and the pile is placed in a multi-plate press that provides heat and pressure simultaneously.
At the end of the heating cycle, which also includes a cooling stage, a compact and rigid product is formed whose single components are closely associated.
The presses suitable for such production axe complex and give a low output due to their having several plates, to the need to provide heat and pressure at the same time, in precise and clearly-defined sequences, to having to provide, by conduction, even temperatures throughout the various packages in the pile of which only those at the top and bottom will be in contact with heating plates. The presence of several heating plates not only complicates the structure but also makes loading and unloading a lengthy process, also creating problems for short production runs due to difficulties over amortizing costs. In particular the penetration of heat from the plates to the piled up packages is hindered because of the distance the heat must travel to reach the innermost packages. High costs are therefore encountered both for plant and for running it, for wasted material, long processing times, all of which means an expensive product. To eliminate or reduce these drawbacks various processes have been devised all based on endothermic heating by means of electric resistances created by just those metal elements needed to form the piles of packages to make plastic laminates, when said piles are put inside presses or in autoclaves to execute the pressing process.
PCT IT92/00101 discloses a process whereby endothermic heating is obtained by forming the metal laminae from a continuous band and connecting said band, especially one of copper, to a source of electric current.
PCT IT92/00097 discloses a process in accordance with which the packages in the pile are made of a multi-composition band and therefore comprise both bands of prepreg and one or two metal bands, especially of copper. The metal bands are connected to a source of electricity. IT MI93 A 000223 discloses a process according to which the electric resistances are obtained by connecting to a source of electricity either the copper lamine or laminae of other material forming the laminates, or the sheets of steel or other metal inserted between one package and another in the pile, or else the series of lengths of electricity conductor bands, bent in a "U", whose ends are inserted between one pair of said steel sheets and the next one, connected together serpentine-wise. IT MI93 A 000224 discloses a process whereby endothermic heating is secured by the metal laminae consisting of two continuous bands of copper or another material that alternate on the two surfaces of prepreg of the packages placed in a row, and connecting said bands to a source of electric current.
According to these disclosures, having inserted the pile of packages in the press or autoclave, the ends of the metal elements in the packages must be connected to the source of electric current.
Said connections must of course be made allowing for the characteristics of the elements to be connected.
As the case may be, said elements can be the metal laminae in the packages, the metal band or bands laid serpentinewise that form said laminae, the sheets of steel placed at the top and bottom of packages, or others. As the forms and materials of said elements are heterogeneous, as a result electric connections may be complex and require much care and in any case adequate timing and manoeuvering.